As Justin Bieber stepped out of his car in London on his 19th birthday March 1, he looked less the fresh-faced teenage heartthrob who seduced America through YouTube videos and more like a Rikers Island inmate.

The shirtless Canadian strutted into his posh hotel in leather pants sagged so low, they exposed his briefs, and cradled the very bottom of his bottom. The outfit was declared “his most shocking look yet” with the Twitterverse — including actress Olivia Wilde — criticizing his choice of “style.”

“I’m a non-Belieber in this look,” says stylist Christiaan Choy, who has worked with Amar’e Stoudemire and Lamar Odom. “He looks absolutely ridiculous. It doesn’t make sense to hold onto such a ridiculous trend.”

The trend Choy refers to is saggy pants, which started among prison inmates who weren’t allowed to wear belts and went on to dominate the urban hip-hop scene from the early ’90s. The look seemed to have faded — until Bieber resurrected it in spectacular fashion.

“It’s a stage of rebellion. He’s a teenager growing up in front of the world,” says celebrity stylist June Ambrose. “It’s more about where he is mentally. I don’t think this is enough to say it will be trendemic.”

For such a vulgar look, the trend has had staying power since Mark Wahlberg put it on the map.

“Marky Mark wasn’t the first guy to wear his jeans under his butt instead of over it,” says Will Welch, GQ’s senior editor, adding, “but he’s the guy who made it popular thanks to those Calvin Klein ads.”

Eminem ushered the cheeky silhouette into the new millennium, as did basketball players like Allen Iverson — until 2005, when the NBA enforced a dress code requiring business attire.

Still, urban youth kept the look alive — as did rappers like Lil Wayne, who went on a saggy-pants crusade.

“When we saw him with his pants literally holding onto the top of his knees, they’re [basically] $5,000 pants,” says Ambrose with a laugh.

Eventually, even politicians became weary of the trend. In 2008, then-presidential candidate Barack Obama urged young men to “pull up their pants,” and two years later, NY state Sen. Eric Adams erected billboards in his Brooklyn borough, exclaiming: “Stop the Sag!”

At the same time, rappers like Jay-Z and P. Diddy started upping their game, opting for sleek, slim-cut, tailored suits instead of street wear.

“[Jay-Z] grew up hip-hop, in my opinion,” says Ambrose, who has styled Jay-Z and Nas. “And I think young people are getting to a place of refinement.”

Still, Bieber didn’t get that memo.

And fashion experts hope others don’t follow in his footsteps and stoop so low.

“Let’s just say don’t try this at home,” Welch says, “unless home is a 17-bedroom Beverly Hills mansion with three Jacuzzis, two white tigers and a trophy case full of Grammys.”